1. Field of the Invention
The present invention relates to a display device and a method of driving the same, and more particularly the present invention relates to an active matrix display device that is driven by a dot-line inversion driving method in combination with a dot sequential precharge driving method and a method of driving the same.
2. Description of the Related Art
One known driving method of a display device having pixels arranged in a matrix, such as an active matrix liquid crystal display (LCD), is a dot sequential driving method in which pixels are sequentially driven for one line (one row) on a pixel-by-pixel basis. The dot sequential driving method includes a 1H inversion driving method and a dot inversion driving method.
The 1H inversion driving method experiences the following problems. When video signals are written, resistance exists between horizontally adjacent pixels on lines (hereinafter referred to as xe2x80x9cCs linesxe2x80x9d) which distribute a predetermined dc voltage to pixels as a common voltage Vcom, and parasitic capacitance exists at intersections of the Cs lines and signal lines. This causes the video signals to jump over onto the Cs lines or gate lines, resulting in oscillations in the potentials of the Cs lines toward the same polarity as those of the video signals. Therefore, significant horizontal crosstalk or defective shading occurs, leading to significant degradation of the picture quality.
When the pixels maintain pixel information in a period of one field, the potentials of the signal lines oscillate every one horizontal scanning period (1H). In the 1H inversion driving method, the polarities of the video signals written to horizontally adjacent pixels are the same, and the potentials of the signal lines increasingly oscillate. This potential oscillation jumps over to the pixels due to the source-drain coupling of pixel transistors, causing significant vertical crosstalk, and resulting in degradation of the picture quality.
On the other hand, in the dot inversion driving method, video signals having opposite polarities are concurrently written to horizontally adjacent pixels and the potential oscillations of the Cs lines or the signal lines are cancelled out between the adjacent pixels, thereby solving the degradation problem of the picture quality exhibited by the 1H inversion driving method. However, since the polarities of the video signals written to horizontally adjacent pixels are opposite, the fields of the adjacent pixels produce domains (optically dropped regions) at the edges of apertures in the pixels. As a result, the aperture ratio of the pixels is reduced, thus providing a lower transmittance and leading to a reduction in contrast.
In order to address such a deficiency, there has been proposed a driving method termed the xe2x80x9cdot-line inversion driving methodxe2x80x9d in which video signals having polarities opposite to each other are concurrently written to two odd-numbered rows of pixels that are spaced apart, e.g., two rows apart vertically, in adjacent pixel columns so that the polarities of horizontally adjacent pixels are the same while the polarities of vertically adjacent pixels are opposite in the array of pixels to which the video signals have been written.
In the dot-line inversion driving method, video signals having opposite polarities are applied to adjacent signal lines, as in the dot inversion driving method, and the polarities .of horizontally adjacent pixels are the same in the array of pixels to which the video signals have been written, as in the 1H inversion driving method. Therefore, degradation of the picture quality due to horizontal crosstalk or shading can be prevented without having to reduce the aperture ratio of the pixels.
However, when video signals written to pixels are inverted every 1H during the dot sequential driving, a significant charging/discharging current when the video signals are written to the signal line extending along each column of pixels appears as vertical fringes on the display screen. In order to reduce the charging/discharging current during the writing of the video signals as much as possible, a precharge driving method has been adopted in which precharge signals are written in advance before the video signals are written.
In general, gray levels are most likely to produce visible vertical fringes. Therefore, the precharge signal level is typically set at a gray level which is most likely to produce visible vertical fringes. If the precharge signal level is set at a gray level, however, vertical crosstalk occurs when a window pattern and the like are displayed, because the amount of source-drain optical leakage of pixel transistors differs according to location from picture to picture, and results in degradation of the picture quality.
In order to prevent such vertical crosstalk, the precharge signal level should be set at the black level, thereby making the source-drain leakage current of the pixel transistors uniform over the entire screen. If the precharge signal level is set at the black level, however, vertical fringes, as previously described, again appear. In summary, vertical crosstalk and vertical fringes are in a trade-off relation.
Accordingly, a 2-step dot sequential precharge method has been proposed in which a black-level signal and a gray-level signal are precharged in two steps. FIG. 8 illustrates a circuit structure of a precharge driving circuit 100 in the active matrix liquid crystal display driven by the 2-step dot sequential precharge method.
In FIG. 8, the precharge driving circuit 100 includes a shift register 101 and a precharge switching circuit 102. When a precharge start pulse PST is input, the shift register 101 shifts or transfers the precharge start pulse PST in turn to shift stages (S/Rs) in synchronization with horizontal clocks HCK and HCKX having opposite phases to each other, and successively outputs it as precharge control pulses PCC1, PCC2, and so on from the shift stages.
The precharge control pulses PCC1, PCC2, etc. are supplied to the precharge switching circuit 102. The precharge switching circuit 102 also receives an odd-column precharge black signal PsigBo via a precharge signal line 103o, an even-column precharge black signal PsigBe via a precharge signal line 103e, an odd-column precharge gray signal PsigGo via a precharge signal line 104o, and an even-column precharge gray signal PsigGe via a precharge signal line 104e. 
In the precharge switching circuit 102, a precharge switch 106-1b is connected between a signal line 105-1 of a pixel section and the precharge signal line 103o, a precharge switch 106-1g is connected between the signal line 105-1 and the precharge signal line 104o, a precharge switch 106-2b is connected between a signal line 105-2 of the pixel section and the precharge signal line 103e, and a precharge switch 106-2g is connected between the signal line 105-2 and the precharge signal line 104e. Other precharge switches are further connected in the same way.
The precharge control pulses PCC1, PCC2, etc. that are output from the shift stages of the shift register 101 are used as drive signals of the precharge switches 106-1b, 106-1g, 106-2b, 106-2g, etc.
Specifically, the precharge control pulse PCC1 from the first stage is applied to the precharge switch 106-1b as a switch driving pulse PSD1b, the precharge control pulse PCC3 from the third stage is applied to the precharge switch 106-1g as a switch driving pulse PSD1g, the precharge control pulse PCC2 from the second stage is applied to the precharge switch 106-2b as a switch driving pulse PSD2b, and the precharge control pulse PCC4 from the fourth stage is applied to the precharge switch 106-2g as a switch driving pulse PSD2g. Other precharge control pulses are further applied in the same way to the subsequent precharge switches.
FIG. 9 is a timing chart of the precharge start pulse PST, the horizontal clock HCK, the black-level switch driving pulses PSD1b, PSD2b, PSD3b, PSD4b, and PSD5b, and the gray-level switch driving pulses PSD1g, PSD2g, PSD3g, and PSD4g. 
If black windows or black lines are displayed on an active matrix liquid crystal display driven by the dot-line inversion driving method in combination with the dot sequential precharge driving method, so-called trails in which black lines appear over and along the horizontal scan direction (hereinafter referred to as xe2x80x9chorizontal trailsxe2x80x9d) occur on circumscribing portions thereof having a higher contrast in intensity, as shown in FIG. 10. Such horizontal trails can degrade the picture quality. The cause of horizontal trails is described as below.
In the dot-line inversion driving method, as previously described, the polarity of the input video signal is inverted from positive to negative or vice versa at odd columns and even columns of pixels with reference to the common voltage Vcom that is commonly supplied to the pixels and is also inverted every 1H. The resulting polarities of the pixel potentials are shown in FIG. 11, in which pixel potentials which are higher and lower than the common voltage Vcom are indicated by H and L, respectively.
If black windows or black lines are displayed, the pixel potentials shown in FIG. 12 are input to the circumscribing portions thereof. In FIG. 12, G represents the gray level and B represents the black level.
FIG. 13 depicts how the potentials of the signal lines vary when the 2-step dot sequential precharge driving method is considered.
In this illustration, as an example, the H and L levels of the precharge gray signals are set at 10 V and 5 V, respectively, and the H and L levels of the precharge black signal are set at 13 V and 2 V, respectively. In a pixel signal, typically, the H and L levels of the gray signal are 9 V and 6 V, respectively, and the H and L levels of the black signal are 13 V and 2 V, respectively.
Referring to FIG. 13, apparently, the potential of the signal line for an odd column varies in the following order: gray L level of an N-th stage pixel potential; precharge black H level; precharge gray H level; and, black H level of an (N+1)-th stage pixel potential. The potential of the signal line for an even column varies in the following order: black H level of an N-th stage pixel potential; precharge black L level; precharge gray L level; and, black L level of an (N+1)-th stage pixel potential.
In this illustration, the potential variations from the N-th stage pixel potential to the precharge black signal level are +7 V for an odd column and xe2x88x9211 V for an even column, and the potential variations cannot be therefore offset. Due to the presence of a potential difference between an odd column and an even column, horizontal trails occur, as described above. Generally, the potential variations of the signal lines are coupled through the parasitic capacitance to gate lines that are connected to rows of gate electrodes of pixel transistors or to Cs lines which distribute the common voltage Vcom to the pixels.
Therefore, if black windows or black lines are displayed using the pixel potentials, as shown in FIG. 12, the coupling cannot be offset between an odd column and an even column, causing the oscillations to be carried on the gate lines and the Cs lines. The oscillations are applied to other pixels as well as those in window bands when the video signals are written, thereby causing horizontal trails of the windows.
Accordingly, it is an object of the present invention to provide a display device driven by a dot-line inversion driving method in combination with a dot sequential precharge driving method which, if black windows or black lines are displayed, is free of horizontal trails on circumscribing portions thereof, and to provide a method of driving the same.
To this end, according to the present invention, a display device includes a pixel section having pixels arranged in a matrix, a signal line extending along each column of pixels, and a gate line extending across two odd-numbered rows that are spaced apart in adjacent pixel columns. The display device further includes a first driving unit for applying scan pulses to the gate lines while scanning the pixels of the pixel section in the row direction, a second driving unit for sequentially providing video signals having opposite polarities to adjacent pixels via the signal lines, the pixels being connected to the gate lines to which the scan pulses are applied by the first driving unit, and a third driving unit. The third driving unit first provides constant level precharge signals together in the horizontal blanking periods before the video signals having opposite polarities are applied to the signal lines by the second driving unit, and then sequentially provides a black-level precharge signal and a predetermined color level precharge signal. The black-level precharge signal has the same polarity as that of one of the video signals, and the predetermined color level precharge signal has the same polarity as that of the other video signal.
When a horizontal scan is performed by the second driving unit, the third driving unit may first provide constant level precharge signals together to the pixels that are selected through a vertical scan performed by the first driving unit in the horizontal blanking periods before video signals having opposite polarities are supplied to the signal lines. Then, the third driving unit may sequentially provide a black-level precharge signal having the same polarity as that of one of the video signals and a predetermined color level precharge signal having the same polarity as that of the other video signal. Subsequently, the second driving unit may provide the video signals having opposite polarities to the signal lines.